Conventionally, a power supply circuit for a discharge lamp, which is more usually referred to as a ballast circuit by those familiar with this technical field, comprises a converter for converting current from direct current to alternating current, together with a starting circuit for the lamp. One example of a discharge lamp power supply circuit is described in European patent specification No. 0 567 408, to which reference can with advantage be made. However, a power supply circuit of the same type is also shown in FIG. 1 of the drawings accompanying the present specification, in which the converter is indicated at 1 and the starting circuit at 2. FIG. 1 also shows a battery 3 which supplies the converter 1 with voltage, and the discharge lamp is indicated at LAD.
The converter 1 maintains the arc of the discharge lamp LAD in an active or operating mode once the lamp has been started, and after a phase of heating of its electrodes has taken place. To this end, the converter supplies an alternating current which is regulated in such a way that the power transmitted to the lamp will be substantially constant.
The function of the starting circuit 2 is to generate a high tension pulse (of the order of 12 to 25 kV) which fires the lamp and sets up the electrical arc in the latter. The starting circuit 2 generally comprises, as shown in FIG. 1, a transformer 4, the secondary winding of which, indicated at 4b, is connected in series with the discharge lamp LAD between the output terminals of the converter 1. The primary winding of the transformer 4 is indicated at 4a, and this is connected in series with a capacitor 5 and a spark gap type device 6.
The capacitor 5 becomes charged until the spark gap 6 fires to produce a spark. When this happens, the capacitor 5 discharges into the primary winding 4a of the transformer, and generates in the secondary of the latter the high tension pulse required.
In another version, it is also known to replace the spark gap device with a controlled interrupter, the capacitor 5 then being not necessarily present.
Whether the primary circuit comprises a spark gap device or a controller interrupter, the transformer of the starter circuit must be of such parameters that it permits generation of a pulse which does not have too short a duration, and it must also ensure good coupling with the secondary of the transformer. The duration of the pulse should be 50 ns or greater, in order to avoid ionization effects and electromagnetic radiation due to the high frequencies. The coupling with the secondary winding should be such that voltage losses are avoided which would otherwise impose increased load voltages on the primary winding and increase the dimensions of the spark gap device or interrupter.
For this reason, the windings of the primary and the secondary have a large number of turns, so that the inductance in the secondary of the starter circuit is generally high, being in particular of the order of 1 mH or higher.
Such an inductance is also acceptable in the case where, when an arc is in operation in the discharge lamp, the alternating current transmitted to the lamp has a nominal frequency of the order of a few kilohertz.
On the other hand, at higher frequencies, which are desirable in particular so as to enable the dimensions of transformers used in the converters to be reduced, this reactive impedance has a voltage dividing effect across the terminals of the discharge lamp. This calls for an increase in the voltage at the output of the converter, and therefore requires the converter to be made larger in size. In addition, the high frequency current that flows through the inductance of the secondary is then the source of major losses in the transformer.